1. Field of the Invention
The inventions disclosed and taught herein relate generally to oil and gas drilling and production equipment; and more specifically relate to an improved design and installation method for belly strakes useful for stabilizing floating, deep-water offshore oil and gas drilling and production platforms.
2. Description of the Related Art
Offshore oil and gas drilling and production operations can involve the provision of a vessel, or platform, sometimes called a rig, on which the drilling, production and storage equipment, together with the living quarters of the personnel manning the platform, if any, may be mounted. In general, offshore platforms fall into one of two classes, that is, “fixed” and “floating” platforms. Fixed platforms often comprise an equipment deck supported by legs that can be seated directly or indirectly on the sea floor. While relatively stable, they are typically limited to relatively shallow waters, e.g., depths of about 500 feet (approximately 152 m) and less. However, at least one so-called “compliant piled tower” (CPT) platform, which is referred to as the “Baldpate” tower, is said to be operating at a depth of 1648 ft. (approximately 500 m).
Floating platforms are typically employed in water depths of about 500 ft. (approximately 152 m) and greater, and may be held in position over the well site by, as examples, mooring lines anchored to the sea floor, motorized thrusters located on the sides of the platform or both. Although floating platforms may be more complex to operate because of their movement in response to environmental conditions, such as wind and water movement, they are generally capable of operating in substantially greater water depths than are fixed platforms. Floating platforms may also be more mobile, and hence, easier to move to other well sites. There are several different types of known floating platforms, such as, for example, so-called “drill ships,” tension-leg platforms (TLPs), semi-submersibles, and spar platforms.
Spar platforms, for example, comprise long, slender, buoyant hulls that give them the appearance of a column, or spar, when floating in an upright, operating position, in which an upper portion extends above the waterline and a lower portion is submerged below it. Because of their relatively slender, elongated shape, they have relatively deeper drafts, and hence, substantially better heave characteristics, e.g., much longer natural periods in heave, than other types of platforms. Accordingly, spar platforms have been thought of by some as a relatively successful platform design over the years. Examples of spar-type floating platforms used for oil and gas exploration, drilling, production, storage, and gas flaring operations may be found in the patent literature in, e.g., U.S. Pat. No. 6,213,045 to Gaber; U.S. Pat. No. 5,443,330 to Copple; U.S. Pat. Nos. 5,197,826; 4,740,109 to Horton; U.S. Pat. No. 4,702,321 to Horton; U.S. Pat. No. 4,630,968 to Berthet et al.; U.S. Pat. No. 4,234,270 to Gjerde et al.; U.S. Pat. No. 3,510,892 to Monnereau et al.; and U.S. Pat. No. 3,360,810 to Busking.
Despite their relative success, spar-type platforms include some aspects that need improvement. For example, because of their elongated, slender shape, they can be relatively more complex to manage during offshore operations under some conditions than other types of platforms in terms of, for example, control over their trim and stability. In particular, because of their elongated, slender shape, spar platforms may be particularly susceptible to vortex-induced vibrations (VIV), which may result from strong water currents acting on the hull of the platform. The provision of apparatus on the elongated hulls for vortex breaking, or controlled vortex-shedding, can reduce or eliminate this problem. For example, U.S. Pat. Nos. 6,148,751 and 6,349,664, to Brown et al., describe a “system for reducing hydrodynamic drag and VIV” for fluid-submersed hulls. U.S. Pat. No. 6,244,785, to Richter et al., describes a “precast, modular spar system having a cylindrical open-ended spar.” Such prior art helical strakes typically can comprise very heavy, helically-formed, edge-supported plates that must be attached, e.g., by welding, to the hull while it is being fabricated, is such as in a dry dock. Moreover, some spar may require belly strakes. When a spar has been built in a fabrication yard, three possibilities may typically be employed for bringing the spar to the offshore site. The first possibility may include towing the spar on the surface of the water, such as with tug boats, for a “wet tow” transport. In this case, the belly strakes may be installed around the hull if the draft of the hull plus the strake panel width does not exceed the yard and the ship channel water depth, normally 45 ft. (14 m). However, sometimes the draft in the yard and/or ship channel may be low, which may make it difficult or impossible to have the fully extended strakes around the hull. The second possibility may include towing the spar on a Heavy Lift Vessel (“HLV”) for a “dry tow” transport. In this scenario, it may not be possible to install the full strakes around the hull, for example, because the hull may have to be maintained on the deck of the barge by a set of supports. Generally, then, the strakes may be installed around a portion of the hull, but not on the part of the hull maintained by the supports. When the barge arrives at the installation site, it may ballasted and the spar may be allowed to float on the surface of the water. The spar may be upended from the horizontal position to a vertical position, wherein finally the rest of the strakes may be installed on the hull. The third possibility may be a combination of the first two possibilities. First, the spar hull is dry transported using a HLV from a remote fabrication yard to a near fabrication yard. After float-off in a deep water pit, the final outfitting will be completed in a near fabrication yard. The final outfitting may include removal of dry tow transportation supports and aids, installation of remaining wet tow aids and lightweight survey of the hull. The hull will be wet towed to the offshore site. For a small diameter hull, the belly strakes can be installed in the quayside of the yard by rotating the spar hull. However, for a large diameter hull, the belly stake may need to be installed offshore due to the limited water depth of the ship channel, for example.
The inventions disclosed and taught herein are directed to an improved system and method for designing and installing a belly strake for a spar with a large diameter hull.